The present invention relates to a method and apparatus for coating a moving web of paper carried through an application zone, and in particular to a coating method and apparatus of the trailing blade type wherein recirculation is provided for coating liquid in stagnant areas at side ends of a coating liquid application zone.
Conventional coaters of the trailing blade type includes means for applying coating material to a paper web that is usually supported and carried by a resilient backing roll, together with a flexible doctor blade located some distance from and on the trailing side of the applicator, which serves to level the applied coating. In general, an excess of coating material is applied to the web, and the blade then meters or removes the excess while uniformly spreading the coating onto the web surface.
In recent years, it has become desirable to produce printed papers having a minimal amount of coating, i.e., on the order of about two or three pounds of coating per ream of paper. To achieve low coat weights on conventional trailing blade equipment, it is necessary to increase the pressure of the trailing blade against the web, which results in a high rate of wear on the blade. High blade pressure also increases the possibility of web breakage and streaking caused by foreign particles being caught between the blade and web.
Many conventional coaters inherently employ a relatively long dwell or soak time, which is the time interval between the initial application and final blading of the coating. As a result, the water portion of the coating composition, as well as the water soluble or dispersible materials contained therein, migrate into the moving web at a more rapid rate than the pigment and eventually cause an undesirable imbalance in the coating constituents and their rheological properties. Long soak periods are also incompatible with the application of successive wet coats without intervening drying because the successive coats tend to migrate into and contaminate the previous coat.
In overcoming the disadvantages of the prior art there has been developed an improved applicator, generally known as a short dwell time applicator, as disclosed in Damrau et al U.S. Pat. No. 4,250,211, assigned to the assignee of the present invention. That applicator comprises a tapered chamber leading from a supply of coating material to a narrow outlet slot and a doctor blade extending from the trailing side of the slot in contact with the web. The leading edge of the chamber adjacent the slot is closely spaced from the supported web so as to form, in conjunction with the pressurized fluid flowing from the slot, a gap with the web in which a liquid seal is established, and the side ends of the slot are substantially sealed to the web to allow the establishment of the positive liquid pressure of the chamber in a zone of application.
The coater forms an enclosed application zone with the web to apply a continuous narrow strip or band of pressurized coating material thereon, which enables application of lower coat weights than have heretofore been feasible. However, because coating liquid is introduced through the slot into the application zone along the entire length of the zone, it is also directed against edge seals at side ends of the zone which seal the zone and enable uncoated margins to be maintained on the edges of the web, increasing the tendency for some of the coating liquid to force its way past the edge seals and cause coating material stickers on the web edges. In order to block direct impingement of coating liquid against the edge seals and reduce leakage therepast, deckles have recently been provided across the slot beneath the seals. However, while such deckles have proven effective in controlling leakage of coating liquid past the edge seals, they create stagnant areas of coating liquid at side ends of the application zone, resulting in an accumulation of stagnate coating thereat and a need to frequently disassemble the applicator for cleaning.